Noise reduction apparatus in an FM stereo tuner

ABSTRACT

A noise reduction apparatus in an FM stereo tuner including a circuit for generating a stereo sum signal and a stereo differential signal; a noise eliminating circuit for dividing, in accordance with several frequency bands, the stereo differential signal into several divisional signals and for outputting, in accordance with signals levels corresponding to the frequency bands, a composite signal of divisional stereo differential signals respectively corresponding to the signal levels; and a circuit for reproducing a stereo signal from the composite signal and the stereo sum signal. In another embodiment, the noise reduction apparatus includes a circuit for generating a stereo sum signal and a first stereo differential signal; a noise eliminating circuit for dividing the stereo sum signal and the first stereo differential signal into several division differential signals and several divisional sum signals, for eliminating a low-level divisional stereo differential signal in each of the frequency bands to generate a second stereo differential signal, and for outputting a composite signal representing the second stereo differential signal and those divisional stereo sum signals in the same divisional band as the eliminated divisional stereo differential signals; and a circuit for reproducing a stereo signal from the composite signal and the stereo sum signal.

FIELD OF THE INVENTION

The present invention relates to a noise reduction apparatus for an FMstereo tuner.

BACKGROUND OF THE INVENTION

A conventional circuit for suppressing noise produced in reception ofstereo broadcasting is known as a Hi-blend circuit. One such Hi-blendcircuit is disclosed in Japanese patent Post-Examination No. 45-32282.

Using this Hi-blend circuit, in the case of FM stereo broadcasting, twohalves of a noise component N of a stereo differential signal (L-R) in acomposite signal exist in demodulated L and R signals, respectively.These two halves of the noise component N exist in such a state that thehalves are in antiphase to each other. In this circuit, only the highband components of the L and R signals are added to each other, therebycancelling the antiphase halves of the noise component N with eachother, and eliminating jarring noise in a high band component.

Although the above-mentioned conventional Hi-blend circuit has a noisereduction effect on high band signal components, this circuit does notact on middle and low band signal components. Accordingly there has beena problem with the conventional Hi-blend circuit in that noise cannot beeliminated over a broad range from a low band to a high band. Moreover,there has been a further problem in that it is impossible to obtain asatisfactory stereo sense because a stereo signal becomes perfectlymonaural in a high band with the Hi-blend circuit so that separation ofthe right and left signals is deteriorated.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to solve theforegoing problems in the prior art.

It is another object of the present invention to provide a noisereduction apparatus in an FM stereo tuner in which a noise component canbe substantially eliminated over the entire frequency band withoutdeteriorating separation.

In order to attain the foregoing objects, the noise reduction apparatusin an FM stereo tuner comprises: a circuit for generating a stereo sumsignal and a stereo differential signal; a noise eliminating circuit fordividing, in accordance with a plurality of frequency bands, the stereodifferential signal into a plurality of divisional differential signalsand for outputting in accordance with signal levels respectivelycorresponding to the plurality of frequency bands, a composite signal ofthe divisional stereo differential signals; and a circuit forreproducing a stereo signal from the composite signal and the stereo sumsignal.

In another embodiment of the invention, the noise reduction apparatus inan FM stereo tuner comprises: a circuit for generating a stereo sumsignal and a first stereo differential signal; a noise eliminatingcircuit for dividing, in accordance with a plurality of frequency bands,the stereo sum signal and the first stereo differential signal into aplurality of divisional differential signals and a plurality ofdivisional sum signals, for eliminating a low-level divisional stereodifferential signal in each of the plurality of frequency bands so as togenerate a second stereo differential signal, and for outputting acomposite signal representing one of the second stereo differentialsignal and a delayed divisional stereo sum signal in each of theplurality of frequency bands; and a circuit for reproducing a stereosignal from the composite signal and the stereo sum signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will be apparentfrom the following description taken in connection with the accompanyingdrawings, wherein:

FIG. 1 is a schematic block diagram for explaining the principle of afirst aspect of the present invention;

FIG. 2 is a schematic block diagram for explaining the principle of asecond aspect of the present invention;

FIG. 3 is a block diagram of a first embodiment according to the presentinvention;

FIG. 4 is a block diagram of a second embodiment according to thepresent invention;

FIG. 5 is a block diagram of a third embodiment according to the presentinvention;

FIG. 6 is a block diagram of a fourth embodiment according to thepresent invention;

FIG. 7 is a block diagram of a fifth embodiment according to the presentinvention;

FIG. 8 is a block diagram of a sixth embodiment according to the presentinvention;

FIG. 9 is a block diagram of a seventh embodiment according to thepresent invention;

FIG. 10 is a block diagram of an eighth embodiment according to thepresent invention;

FIG. 11 is a block diagram of a ninth embodiment according to thepresent invention; and

FIG. 12 is a block diagram of a tenth embodiment according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the principle of a first aspect of the present invention will begenerally described.

FIG. 1 is a block diagram generally showing a first aspect of thepresent invention. As shown in FIG. 1, the FM stereo tuner according tothe first aspect of the present invention comprises: a circuit 2 forgenerating a stereo sum signal (L+R) and a stereo differential signal(L-R); a noise eliminating circuit 3 for dividing the stereodifferential signal (L-R) into a plurality of frequency bands and foroutputting a composite signal (L-R)' of divisional stereo differentialsignals (L-R)_(d) in accordance with signals levels of the respectivedivisional bands; and a circuit 4 for reproducing a stereo signal (R,L)from the composite signal (L-R)' and the stereo sum signal (L+R).

According to the first aspect of the present invention, the stereodifferential signal (L-R) produced from the circuit 2 is supplied to thenoise eliminating circuit 3 so as to be subjected to noise eliminationprocessing.

The noise eliminating circuit 3 divides the stereo differential signal(L-R) into a plurality (n) of frequency bands, performs noiseelimination processing in each of the divisional frequency bands byextracting the divisional stereo differential signal (L-R)_(d) inaccordance with the signal level of each divisional frequency band, andoutputs a composite signal of those divisional stereo differentialsignals (L-R)_(d), that is, a composite signal (L-R)' in which a noisecomponent is reduced. This composite signal (L-R)' is supplied to thecircuit 4 for reproducing a stereo signal.

The stereo signal reproducing circuit 4 reproduces a stereo signal (L,R)from the input composite signal (L-R)' and the stereo sum signal (L+R).This reproduced stereo signal (L,R) is in a state such that noise iseliminated over all frequency bands, and L and R signals are notcombined, so that it is possible to prevent separation from beingdeteriorated.

FIG. 2 is a block diagram generally showing a second aspect of thepresent invention. As shown in FIG. 2, the FM stereo tuner according toa second aspect of the present invention comprises: a circuit 2 forgenerating a stereo sum signal (L+R) and a stereo differential signal(L-R); a noise eliminating circuit 3 for dividing the stereo sum signal(L+R) and the stereo differential signal (L-R) into a plurality offrequency bands with the same dividing frequencies respectively, foreliminating a low-level divisional stereo differential signal (L-R)_(d)in each divisional band so as to generate a second stereo differentialsignal, and for outputting a composite signal of the second stereodifferential signal and a delay signal (L+R)' of a divisional stereo sumsignal in the same divisional band as the eliminated divisional stereodifferential signal (L-R)_(d) ; and a circuit 4 for reproducing a stereosignal (L,R) from the composite signal and the stereo sum signal (L+R).

According to the second aspect of the present invention, the circuit 2for generating a stereo sum signal and a stereo differential signalgenerates a stereo sum signal (L+R) and a stereo differential signal(L-R). The generated stereo sum signal (L+R) and stereo differentialsignal (L-R) are divided into a plurality of frequency bands with thesame divisional frequencies respectively, and are subject to noiseelimination processing by a noise eliminating circuit 3.

The noise eliminating circuit 3 eliminates a low-level divisional stereodifferential signal in each divisional band and generates the secondstereo differential signal. Although this second stereo differentialsignal is improved in a signal-to- noise ratio, its signal level isreduced by the degree of the eliminated noise component in comparisonwith the stereo differential signal (L-R) provided from the circuit 2.In the noise eliminating circuit 3, however, the delay signal (L+R)' ofthe divisional stereo sum signal in the same divisional band as that ofthe eliminated divisional stereo differential signal (L-R)_(d) is addedto the second stereo differential signal so as to generate a compositesignal, so that it is possible to compensate for the loss of the stereodifferential signal caused by the limitation of the noise component. Asa result, the level of the composite signal fed to the stereoreproducing circuit 4 is not deteriorated, and it is possible to preventdeterioration in a stereo sense of the stereo signal (L,R) reproducedfrom the composite signal and the stereo sum signal (L+R) by means ofcircuit 4.

Embodiments of the present invention will now be described withreference to the drawings.

First, various embodiments with respect to the first aspect of thepresent invention as illustrated in FIG. 1 will be described.

FIG. 3 shows a first embodiment of the present invention.

In FIG. 3, a noise eliminating circuit 3A includes: a plurality of bandpass filters (BPF_(l) through BPF_(n)) 7_(-l) through 7_(-n) fordividing a stereo differential signal (L-R) supplied from a stereo sumsignal and stereo differential generating circuit 2 into a plurality (n)of audible frequency bands; switch control circuits (SWC_(l) throughSWC_(n)) 8_(-l) through 8_(-n) for outputting switch control signalsA_(l) through A_(n) correspondingly to divisional stereo differentialsignals (L-R)_(d) provided from the respective band pass filters 7_(-l)through 7_(-n) ; switch circuits 9_(-l) through 9_(-n) for controllingpassing of the divisional stereo differential signals (L-R)_(d) on thebasis of the switch control signals A_(l) through A_(n) ; and an adder10 for outputting a composite signal (L-R)' representing the sum of theoutputs of the switch circuits 9_(-l) through 9_(-n) ; and divisionalstereo differential signals (L-R)_(d) passed through the respectiveswitch circuits 9_(-l) through 9_(-n). The switch control circuits8_(-l) through 8_(-n) have reference level signals V_(thl) throughV_(thn), respectively, whose reference level signals correspond to noisecomponent levels included in the respective divisional stereodifferential signal (L-R)_(d). The switch control circuits 8_(-l)through 8_(-n) compare the divisional stereo differential signals(L-R)_(d) with their reference level signals V_(thl) through V_(thn),respectively, and in response to this comparison, output the switchcontrol signals A_(l) through A_(n) which close the switch circuit9_(-l) through 9_(-n) only when the divisional stereo differentialsignals (L-R).sub. d exceed their reference level signals V_(thl)through V_(thn) respectively. The switch control circuits 8_(-l) through8_(-n) are, for example, constituted by operational amplifiers or thelike. Further, the switch circuits 9_(-l) through 9_(-n) may be, forexample, switching transistors of the like. Further, the adder 10 may bean operational amplifier or the like.

In the above-mentioned configuration, a noise processing circuit isconstituted by the switch control circuits 8_(-l) through 8_(-n) and theswitch circuits 9_(-l) through 9_(-n).

Next, the operation will be described.

A stereo sum signal (L+R) generated from circuit 2 is fed directly tocircuit 4 which reproduces a stereo signal, and a stereo differentialsignal (L-R) is fed to the noise eliminating circuit 3A.

In the noise eliminating circuit 3A, the stereo differential signal(L-R) is divided into n separate signals by the band pass filters 7_(-l)through 7_(-n). The respective divisional stereo differential signals(L-R)_(d) are compared with their reference level signals V_(thl)through V_(thn) in the switch control circuits 8_(-l) through 8_(-n)respectively. As a result of the comparison, the switch control signalsA_(l) through A_(n) each has a binary value of ON/OFF are put out onlywhen the condition

    V.sub.thl ˜V.sub.thn <(L-R).sub.d

is satisfied. That is, the condition

    V.sub.thl ˜V.sub.thn <(L-R).sub.d

means the case where a signal component of each divisional stereodifferential signal (L-R)_(d) is larger than a noise component level inthe respective divisional band. In response to the switch controlsignals A_(l) through A_(n), the switch circuits 9_(-l) through 9_(-n)close (turn ON) their switches to pass the divisional stereodifferential signals (L-R)_(d) at that time. On the other hand,therefore, the other stereo differential signals (L-R)_(d) in thedivisional bands under the condition of

    V.sub.thl ˜V.sub.thn ≧(L-R).sub.d,

are prevented from being outputted because the switch circuits 9_(-l)through 9_(-n) are opened (turned OFF).

The respective divisional stereo differential signals (L-R)_(d) whichhave passed are added to each other in the adder 10 and supplied as acomposite signal (L-R)' to the circuit 4 for reproducing a stereosignal. Then, since the composite signal (L-R)' is composed of only thedivisional stereo differential signals (L-R)_(d) each having a low noiselevel, the composite signal (L-R)' is reduced in its noise component incomparison with the stereo differential signal (L-R) put out from thecircuit 2 for generating a stereo sum signal and a stereo differentialsignal.

The circuit 4 reproduces a stereo signal from the stereo sum signal(L+R) and the composite signal (L-R)' thus reduced in its noisecomponent, thereby generating L and R signals. It is therefore possibleto reduce a noise component included in the generated L and R signals.

FIG. 4 shows a second embodiment of the present invention.

This embodiment has a feature in that in a noise eliminating circuit 3B,each level of the reference level signals V_(thl) through V_(thn) inswitch control circuits 11_(-l) through 11_(-n) is controlled variablyin accordance with the level of the electric field intensity in thelocation where an FM tuner is installed.

The embodiment of FIG. 4 is different from the FIG. 3 embodiment in thatan electric field intensity level signal SML is led in through an inputterminal 12 and branchingly fed to switch the control circuits 11_(-l)through 11_(-n). Items which are the same as or equivalent to those inFIG. 3 are referenced correspondingly and the descriptions about thoseitems are omitted here as being redundant.

In this embodiment, the switch control circuits 11_(-l) through 11_(-n)have signal generators (not shown) for generating the reference levelsignals V_(thl) through V_(thn), and the electric field level signal SMLis fed to the generators so that the generators generate the referencelevel signals V_(thl) through V_(thn) (corresponding to the electricfield level signal SML) to reference input terminals of the comparingportions of the switch control circuits respectively.

The reason why such a configuration is made will be described. A noisecomponent of a divisional band included in a stereo differential signal(L-R) varies in accordance with the electric field intensity. If theelectric field intensity is low, then the noise level is high relativeto the stereo differential signal (L-R), while it is low if the electricfield intensity is high. Taking the reception state in various electricfield intensive areas into consideration, therefore, there is apossibility that the noise eliminating effect becomes insufficient ifall the reference level signals V_(thl) through V_(thn) are establisheduniformly. In this embodiment, accordingly, the reference level signalsV_(thl) through V_(thn) are controlled automatically in accordance withthe level of the electric field intensity, so that it is possible torealize a desired noise elimination on the basis of the reference levelsignals V_(thl) through V_(thn).

Next, the operation will be described.

An electric field level signal SML is fed to the switch control circuits11_(-l) through 11_(-n). The respective switch control circuits 11_(-l)through 11_(-n) set the reference level signals V_(thl) through V_(thn)in accordance with the input electric field level signal SML, comparethe divisional stereo differential signals (L-R)_(d) with the referencelevel signals V_(thl) through V_(thn) respectively, feed the switchcontrol signals A_(l) through A_(n) to the switch circuits 9_(-l)through 9_(-n) in the same manner as in FIG. 3, and output thedivisional stereo differential signals (L-R)_(d) only when they arehigher than their respective reference level signals, so as to obtain acomposite signal (L-R)'.

FIG. 5 shows a third embodiment of the present invention.

This embodiment has a feature in that in a noise eliminating circuit 3C,instead of using the switch control circuits 8_(-l) through 8_(-n) ofFIG. 3 which output the switch control signals A_(-l) through A_(-n)each of which has a binary value of ON/OFF, this embodiment includescontrol circuits 15_(-l) through 15_(-n) which output control signalsB_(l) through B_(n) corresponding to the quantity of deviation betweenthe reference level signals V_(thl) through V_(thn) and the divisionalstereo differential signals (L-R)_(d), and instead of using the switchcircuits 9_(-l) through 9_(-n) of FIG. 3, this embodiment includesvariable level adjusting circuits 13_(-l) through 13_(-n) which may bevoltage-controlled amplifiers or the like. Other elements shown in FIG.5 which are the same as or equivalent to, those in FIG. 3 are referencedcorrespondingly, and the descriptions about those items are omitted.

Each signal level of the passing divisional stereo differential signals(L-R)_(d) is controlled continuously and variably by use of the controlcircuits 15_(-l) through 15_(-n) and the variable level adjustingcircuits 13_(-l) through 13_(-n), so that over the frequency bands,levels of divisional stereo differential signals of omitted frequencycomponents are not extreme and it is therefore possible to reduce anunnatural sense in separation on audition.

FIG. 6 shows a fourth embodiment of the present invention.

This embodiment has a feature in that a noise eliminating circuit 3D,based on the third embodiment of FIG. 5 and in the same manner as thesecond embodiment, in order to make it possible that the respectivelevels of the reference level signals V_(thl) through V_(thn) in thecontrol circuits 15_(-l) through 15_(-n) are set variably in accordancewith the electric field intensity in the location where the FM tuner isinstalled. An electric field level signal SML is supplied from the inputterminal 12 and branchingly fed to the respective control circuits16_(-l) through 16_(-n). In FIG. 6, other items which are the same as,or equivalent to, those items in FIG. 5 are referenced correspondingly,and the description about them is omitted.

By the configuration shown in FIG. 6, it is possible to realize asmoother control because it is possible to variably set the referencelevel signals V_(thl) through V_(thn) in accordance with the electricfield intensity, in addition to the variable and continuous control onthe divisional stereo differential signals (L-R)_(d) in accordance withthe quantity of deviation between the levels of noise componentsincluded in the respective divisional bands and the divisional stereodifferential signals (L-R)_(d).

FIG. 7 shows a fifth embodiment of the present invention.

This embodiment has a feature in that in a noise eliminating circuit 3E,a dynamic expander 14 is inserted between the output terminal of theadder 10 and an input terminal of the stereo signal reproduction circuit4. In FIG. 7, other items which are the same as, or equivalent to thosein FIG. 3 are referenced correspondingly, and therefore the descriptionabout these items is omitted.

The reason why the dynamic expander 14 is inserted in this embodimentwill be explained.

When noise elimination is performed in the noise eliminating circuit 3A,the divisional stereo differential signals (L-R)_(d) in the respectivedivisional bands in which the divisional stereo differential signals(L-R)_(d) are low in their signal level are not outputted. Accordinglythe signal level of a composite signal (L-R)' is reduced at least by the"not-passing" divisional stereo differential signals (L-R)_(d), andtherefore the composite signal (L-R)' becomes lower than a stereodifferential signal (L-R) at the time of being outputted from circuit 2.Accordingly, by inserting a dynamic expander 14 in the noise eliminatingcircuit 3E, the level of the composite signal (L-R)' can be recovered tothe level of its original stereo differential signal (L-R), and it istherefore possible to compensate for the deterioration of separation aswell as to eliminate a noise component thereby enlarging the extensionof a stereo sense.

This insertion of the dynamic expander 14 is applicable not only to thefirst embodiment but also to the second, third and fourth embodiments,in each of which the same effect can be obtained.

Next, various embodiments with respect to the second aspect of thepresent invention as illustrated in FIG. 2 will be described.

In FIG. 8, items which are the same as, or equivalent to, those in FIG.3 are referenced correspondingly, and the description about these itemswill be omitted.

The embodiment shown in FIG. 8 has a different noise eliminating circuitthan that of the embodiment of FIG. 3.

A noise eliminating circuit 3A in this embodiment includes a delaycircuit 12 for outputting a signal (L+R)' of a stereo sum signal (L+R)delayed by a predetermined time (hereinafter simply referred to as"delay signal"), and a plurality of noise processing circuits 11_(-l)through 11_(-n) for performing noise processing by use of the delaysignal (L+R)' and the stereo differential signal (L-R) which are fedthereto.

A single delay circuit 12 is provided so that it can be used in commonto the noise processing circuits 11_(-l) through 11_(-n).

Moreover, it is possible to set the signal level of the delayed L+R to adesired value.

The noise processing circuits 11_(-l) through 11_(-n) are made equal innumber to the signal divisional bands which will be described later. Thenoise processing circuits 11_(-l) through 11_(-n) are the same inconfiguration to each other, while they are different from each otheronly in the frequency bands allocated to them respectively. Accordingly,only the noise processing circuit 11_(-l) will be described hereunderand the description about the remainder noise processing circuits 11₋₂through 11_(-n) will be omitted.

The noise processing circuit 11_(-l) is constituted by first and secondband pass filters 13 and 14 forming a signal dividing circuit fordividing a delay signal (L+R)' and a stereo differential signal (L-R)respectively with the same divisional frequencies within an audiblefrequency band, a control circuit 15 for outputting a control signal Acorrespondingly to a divisional stereo differential signal (L-R)_(d) fedfrom the second band pass filter 14, and first and second switchcircuits 16 and 17, the ON/OFF operation of which is controlled by thecontrol signal A.

The control circuit 15 compares the divisional stereo differentialsignal (L-R)_(d) in the divisional band allocated thereto with areference level V_(th) corresponding to the level of a noise componentof the divisional stereo differential signal (L-R)_(d), and outputs acontrol signal A when the divisional stereo differential signal(L-R)_(d) is smaller than the reference level V_(th). This controlcircuit 15 may be constituted, for example, by a comparator using anoperational amplifier.

Upon reception of the control signal A, the first switch circuit 16 isclosed to pass a divisional delay signal (L+R)' fed thereto from thefirst band pass filter 13, and at the same time, the second switchcircuit 17 is opened so as to prevent the passage of a divisional stereodifferential signal (L-R)_(d) fed thereto from the second band passfilter 14. These first and second switch circuits 16 and 17 may beconstituted by switching transistors or the like.

Next, the operation will be described.

A stereo sum signal (L+R) generated from the circuit 2 is fed to thedelay circuit 12 and the circuit 4, and a stereo differential signal(L-R) is fed to the second band pass filter 14.

In the noise eliminating circuit 3A, a delay signal (L+R)', from thedelay circuit 12, is fed to the first band pass filter 13.

In the first and second band pass filters 13 and 14, the delay signal(L+R)' and the stereo differential signal (L-R) are divided with thesame divisional frequencies respectively, and a divisional delay signal(L+R)' and a divisional stereo differential signal (L-R)_(d) aresupplied from the first and second band pass filters 13 and 14,respectively.

If the signal level of the divisional stereo differential signal(L-R)_(d) is larger than the reference level V_(th), the control circuit15 does not output a control signal A, so that the second switch circuit17 passes the divisional stereo differential signal (L-R)_(d) to theadder 10, and the first switch circuit 16 prevents the passage of thedivisional delay signal (L+R)'.

On the other hand, if the signal level of the divisional stereodifferential signal (L-R)_(d) is smaller than the reference level V_(th)(that is, a noise component is larger than a signal component), thecontrol circuit 15 outputs a control signal A to switch the first andsecond switch circuits 16 and 17. As the result, the second switchcircuit 17 prevents the passage of the divisional stereo differentialsignal (L-R)_(d), and the first circuit 16 passes the delay signal(L+R)' to the adder 10. That is, at this time, although the divisionalstereo differential signal (L-R)_(d) is lost, the delay signal (L+R) isfed to compensate for the loss.

The foregoing operation is performed in each of the noise processingcircuits 11_(-l) through 11_(-n), so that their output signals, that is,the divisional delay signals (L+R)' and/or divisional stereodifferential signals (L-R)_(d) are added to each other in the adder 10.The adder 10 feeds the composite signal [(L+R)'+(L-R)_(d) ] to thecircuit 4 which reproduces a stereo signal. This composite signal[(L+R)'+(L-R)_(d) ] means that although a noise component is reduced bythe elimination of low signal-level divisional stereo differentialsignals (L-R)_(d), the delay signals (L+R)' compensate for the lostsignals. Accordingly, the composite signal [(L+R)'+(L-R)_(d) ] isrecovered to a signal level corresponding to that of the original stereodifferential signal fed from the circuit 2.

FIG. 9 shows a seventh embodiment of the present invention.

This embodiment has a feature in that in a noise eliminating circuit 3B,the level of the reference level signal V_(th) in the control circuit 15is controlled variably in accordance with the level of the electricfield intensity in the location where the FM tuner is installed.

FIG. 9 is different from FIG. 8 in that an electric field intensitylevel signal SML is fed to an input terminal 18 so as to be branchinglyfed to each control circuit 15. In FIG. 9, items which are the same as,or equivalent to, those in FIG. 8 are referenced correspondingly andtherefore the description about these items will be omitted.

In this embodiment, each control circuit 15 has a signal generator (notshown) for generating a reference level signal V_(th), and an electricfield level signal SML is fed to each signal generator so that thesignal generator produces the reference level signal V_(th) inaccordance with the electric field level signal SML at a reference inputterminal of a comparing portion.

The reason why such a configuration is made will be described. The levelof a noise component included in a stereo differential signal (L-R)varies in accordance with the electric field intensity. If the electricfield intensity is low, the noise level is high relative to the stereodifferential signal (L-R). while if the electric field intensity ishigh, the noise level is low relative to the stereo differential signal(L-R). Taking the reception state in various electric field intensiveareas into consideration, therefore, there is a possibility that thenoise elimination effect becomes insufficient if all the reference levelsignals V_(th) are established uniformly. In this embodiment, therefore,the reference level signal V_(th) is controlled automatically inaccordance with the level of the electric field intensity, so that it ispossible to realize desired noise elimination by the proper referencelevel signal V_(th).

Next, the operation will be described.

An electric field level signal SML is fed to each control circuit 15.The control circuit 15 sets a reference level signal V_(th) inaccordance with the input electric field level signal SML, compares thedivisional stereo differential signal (L-R)_(d) with the reference levelsignal V_(th), and feeds a switch control signal A to the switchcircuits 16 and 17 in the same manner as in FIG. 8, so as to reduce anoise component and to obtain a composite signal [(L+R)'+(L-R)_(d) ]recovered to its original level.

Next. FIG. 10 shows an eighth embodiment of the present invention.

This embodiment has a feature in that in a noise eliminating circuit 3C,instead of the control circuit 15 of FIG. 8 which outputs a switchcontrol signal A of a binary value (ON/OFF), a control circuit 19 whichoutputs a control signal B corresponding to the quantity of deviationbetween the reference level signal V_(th) and the divisional stereodifferential signal (L-R)_(d) is used, and instead of the switchcircuits 16 and 17, variable level adjusting circuits 20 and 12, forexample, constituted by voltage-controlled amplifiers or the like, forcontinuously and variably controlling the quantity of passage of thedelay signal (L+R)' and the divisional stereo differential signal(L-R)_(d) are employed. In FIG. 10, items which are the same as, orequivalent to, those in FIG. 8 are referenced correspondingly, andtherefore the description about these items will be omitted.

Thus, the signal level of a passing divisional stereo differentialsignal (L-R)_(d) is controlled continuously and variably by use of thecontrol circuit 19 and the variable level adjusting circuits 20 and 21,so that the levels of the omitted frequency components are not extremelyunbalanced over all the divisional frequency bands, and it is thereforepossible to reduce an unnatural sense of separation on audition.

FIG. 11 shows a ninth embodiment of the present invention.

This embodiment has a feature in that in a noise eliminating circuit 3D,based on the eighth embodiment in FIG. 10 and in the same manner as theseventh embodiment, in order to variably set the respective levels ofthe reference level signals V_(th) in the control circuit 19 inaccordance with the electric field intensity in the location where theFM tuner is installed, an electric field level signal SML is led in fromthe input terminal 18 and branchingly fed to each control circuit 19. InFIG. 11, items which are the same as, or equivalent to, those in FIG. 10are referenced correspondingly, and therefore the description aboutthese items is omitted. By such a configuration, it is possible torealize a smoother control because it is possible to variably set thereference level signals V_(th) in accordance with the electric fieldintensity, in addition to the variable and continuous control on thedivisional stereo differential signals (L-R)_(d) in accordance with thequantity of deviation between the levels of noise components included inthe respective divisional bands and the divisional stereo differentialsignals (L-R)_(d).

FIG. 12 shows a tenth embodiment of the present invention.

This embodiment has a feature in that in a noise eliminating circuit 3E,a dynamic expander 22 is inserted between the output terminal of theadder 10 and circuit 4. In FIG. 12, items which are the same as, orequivalent to, those in FIG. 8 are referenced correspondingly, andtherefore the description about these items is omitted.

In this embodiment, the insertion of the dynamic expander 22 makes itpossible to improve separation and therefore to improve the sense ofattendance and sense of stereo.

The insertion of the dynamic expander 22 is applicable not only to thesixth embodiment but also to the seventh to ninth embodiments, in each,the same effect can be obtained.

As has been described above, according to the present invention, astereo differential signal generated by a circuit for generating astereo sum signal and a stereo differential signal is divided into aplurality of frequency bands, only the divisional stereo differentialsignals that have a level higher than a noise level in the divisionalband thereof are composed, and L and R signals are reproduced by acircuit for reproducing a stereo signal, so that substantially withoutdeterioration of separation, a noise component can be eliminated overthe entire frequency bands, and it is possible to reproduce an FM stereobroadcast signal having a natural stereo sense and good tone quality.

Further, as has been described above, according to the presentinvention, a stereo differential signal generated by a circuit forgenerating a stereo sum signal and a stereo differential signal isdivided into a plurality of frequency bands, and a divisional stereodifferential signal lost by a noise eliminating processing performed ineach divisional band is compensated with a delay signal of the stereosum signal, so that it is possible to recover a signal level fed into acircuit for reproducing a stereo signal to the output level of thecircuit for generating a stereo sum signal and a stereo differentialsignal. It is therefore possible to reproduce an FM stereo broadcastsignal having a natural stereo sense and good tone quality.

What is claimed is:
 1. A noise reduction apparatus in an FM stereo tunercomprising:a circuit for generating a stereo sum signal and a firststereo differential signal; a noise eliminating circuit for dividingsaid stereo sum signal and said first stereo differential signal, inaccordance with a plurality of frequency bands, into a plurality ofdivisional differential signals and a plurality of divisional sumsignals, for eliminating a low-level divisional stereo differentialsignal in each of said plurality of frequency bands so as to output asecond differential signal representing those first differential signalsnot eliminated, and for outputting a composite signal representing thesecond stereo differential signal and only those divisional stereo sumsignals in the same divisional band as said eliminated divisional stereodifferential signals; and a circuit for reproducing a stereo signal fromsaid composite signal and said stereo sum signal.
 2. The noise reductionapparatus in an FM stereo tuner according to claim 1, wherein said noiseeliminating circuita dividing circuit for dividing, in accordance withthe plurality of frequency bands, said stereo sum signal and said firststereo differential signal into the plurality of divisional differentialsignals and the plurality of divisional sum signals; a control circuitfor comparing a divisional stereo differential signal in each of saidfrequency bands with a reference level signal corresponding to a noisecomponent level of said divisional stereo differential signal, and foroutputting a control signal for eliminating said divisional stereodifferential signal when said divisional stereo differential signal issmaller than said reference level signal; and a switch circuit forinterrupting said divisional stereo differential signal in response tosaid control signal, and, at the same time, for passing a divisionalstereo sum signal in the same divisional frequency band as saidinterrupted divisional stereo differential signal.
 3. The noisereduction apparatus in an FM stereo tuner according to claim 1, whereinsaid noise eliminating circuit comprises:a dividing circuit fordividing, in accordance with a plurality of frequency bands, said stereosum signal and said first stereo differential signal into a plurality ofdivisional differential signals and a plurality of divisional sumsignals; a control circuit for comparing a divisional stereodifferential signal in each of said frequency bands with a referencelevel signal corresponding to a noise component level of said divisionalstereo differential signal to obtain a deviation therebetween, and foroutputting a control signal corresponding to the quantity of saiddeviation; and a variable level adjusting circuit for reducing the levelof said second stereo differential signal in response to said controlsignal, and for increasing, correspondingly to the reduced level, thelevel of a delay signal of said divisional stereo sum signal belongingto the same divisional band as each of said eliminated stereodifferential signals.
 4. The noise reduction apparatus in an FM stereotuner according to claim 2, wherein said reference level signal of saidnoise eliminating circuit is made variable in accordance with an inputsignal representing an electric field intensity of said FM tuner.
 5. Thenoise reduction apparatus in an FM stereo tuner according to claim 3,wherein said reference level signal of said noise eliminating circuit ismade variable in accordance with an input signal representing anelectric field intensity of said FM tuner.
 6. The noise reductionapparatus in an FM stereo tuner according to claim 1, wherein a dynamicexpander circuit is interposed between said noise eliminating circuitand said stereo signal reproducing circuit.
 7. The noise reductionapparatus in an FM stereo tuner according to claim 2, wherein a dynamicexpander circuit is interposed between said noise eliminating circuitand said stereo signal reproducing circuit.
 8. The noise reductionapparatus in an FM stereo tuner according to claim 3, wherein a dynamicexpander circuit is interposed between said noise eliminating circuitand said stereo signal reproducing circuit.
 9. The noise reductionapparatus in an FM stereo tuner according to claim 4, wherein a dynamicexpander circuit is interposed between said noise eliminating circuitand said stereo signal reproducing circuit.